The invention relates to single-side pigment-coated thin films based on preimpregnates.
Decorative films with a surface finish are decorative surface materials for chipboard and MDF (medium density fibre) board in furniture making. Said decorative films are synthetic resin impregnated or synthetic resin impregnated and subsequently surface-treated, decoratively printed or unprinted paper webs in a state which can no longer be reactivated under pressure and heat. Depending on the type of impregnation process, a distinction is made between thermosetting decorative films with a through-impregnated core (off-line process) and decorative films based on preimpregnates (on-line process), on which a finish coating may then be carried out. This produces a decorative film having a fully finished surface.
Decorative films based on preimpregnates are already impregnated to a greater or lesser depth with a resin mixture within the paper machine, immediately after sheet formation, depending on the mass per unit area of the base paper. After drying, the paper or film must be resistant to splitting so that no fibre separation occurs within the decorative film during the subsequent processing stages of finish coating and laminating onto appropriate supports.
The desired opacity, color and optionally light fastness of said preimpregnates is achieved by adding large quantities of white pigments to the paper pulp.
Preimpregnates or decorative films with a mass per unit area of between 20 and 80 g/m.sup.2 are referred to as thin films. These thin films are mostly characterised by good flatness and can therefore be laminated without difficulty, whereas their ability to be printed with water-based or solvent-based gravure inks satisfies only average requirements in view of the limited smoothing properties due to the synthetic resin content.
Also known are non-impregnated thin films, so-called Japanese paper with a mass per unit area of between 20 and 50 g/m.sup.2 which are increasingly being used as decorative surface materials.
In contrast to preimpregnates, said white or colored paper contains only small proportions of pigments, if any, and therefore has only a low opacity in the untreated state. As a result, it is limited in its ability to mask sufficiently the surface of chipboard or MDF board. Said Japanese papers have very good internal strength and single-side smoothness, these properties making them very receptive to gravure printing and lacquering. On the other hand, said Japanese paper which are not impregnated with synthetic resins but only treated on the surface with an unpigmented polymer film have a very strong tendency to curl at the edges during the laminating process using mostly waterbased adhesives. In the generally conventional continuous process using roller laminating machines, said paper can therefore be fixed to chipboard and MDF board and dried only with difficulty. The board thus covered, however, can then be machined just as easily as when preimpregnates are used since the laminated paper exhibits only short edge tear back.
The object of the invention was, therefore, to provide thin films with the lowest possible mass per unit area which have sufficient laminar strength and good flatness and can be roller-laminated equally well or even more quickly with waterbased adhesives but are additionally characterised by very good gravure printing and lacquering properties. Moreover, the thin films with a comparable mass per unit area should have greater light fastness and opacity, if possible, than conventional Japanese paper and better strength properties than known preimpregnates.
The object is achieved by a thin film which has been provided on the upper side, the printing or lacquering side, with a pigment coating and is impregnated starting from the underside, the laminating side, with a resin.
To prepare the thin films according to the invention, pulp fibre mixtures with high absorbency and strength are used, for example, those made from eucalyptus pulp and pine wood sulphate pulp. A beater sizing is not required in each case, and the pulp fibre mixtures may contain white or colored pigments, depending on the desired application.
The pigment coating formed on one side of the thin film has a thickness of 1 to 8 g/m.sup.2, for example 3, 5 or 7 g/m.sup.2. The pigment to binder ratio in the pigment coating is preferably 1:0.05 to 1:2 and in a particularly preferred embodiment 0.005 to 1:2 or 1:0.08 to 1:0.35 based on the solids content.
For the preparation of the pigment coating white pigments such as clay, calcium carbonate, aluminium hydroxide, talc or titanium dioxide or color pigments such as iron oxide pigments, carbon black, copper, aluminium, other metal pigments, or organic color pigments alone or in mixture, optionally fluid dyes are added. The pigment coating may contain natural or synthetic binders such as starch, polyvinyl alcohol, carboxymethylcellulose, polymer latices on the basis of acrylic acid, acrylic acid ester, styrene, butadiene or vinyl acetate acryle nitrile. According to a preferred embodiment the pigment coating contains 1 to 10 parts by weight based on the amount of pigment of melamine resin, urea resin or phenolic resin or glyoxal resin. Conventional crosslinking agents and penetration aids may be included in the pigment coating.
The pigment coating must have barrier properties with respect to aqueous or solvent-containing substances, such as lacquers and printing inks. In order to obtain good smoothing properties and hence sufficient barrier properties with the thinnest possible pigment coatings, pigments in flake form are particularly preferred, such as clay, aluminium hydroxide or talc. It is also possible, however, to satisfy these properties in large measure by way of a suitable choice of the type and proportion of binders in the pigment coating.
Lowering the mass per unit area of the single-side pigment-coated thin films according to the invention does, however, put limits on the strength and covering power (opacity). On the one hand, the opacity may be increased by adding opacity-increasing, light-fast white or colored pigments to the fibrous material, but at the same time this has an adverse effect on the paper strength. The addition of dry and wet strength agents may offset to a certain extent this adverse effect on the paper strength. On the other hand, during the production of colored paper, it is readily possible to replace said pigments wholly or partially by aqueous dyes with a lower light fastness since the pigment coating is able to reduce the disadvantage of lower light fastness of such colored paper. The desired color of the paper is achieved without sacrificing strength by adding, for example, anionic or cationic direct dyes to the fibrous material, but the opacity and light fastness are improved at the same time by the single side pigment coating.
Aqueous polymer dispersions on the basis of the above mentioned monomers and melamin, urea or phenolic resins as well as mixtures therof with polymer dispersions are used for impregnation of the paper. By the addition of a portion of at most about 10 parts by weight (solid) of pigments to the impregnation liquid opacity of white pre-impregnates still may be increased. Resin uptake may amount to 5 to 30% wt. based on the weight of the base paper, preferably 5 to 15% wt.
Moreover, other auxiliaries may be added to the impregnating mixtures, such as wetting agents, viscosity regulators, anti-adhesion agents and penetrating agents, pigments, dyes and defoamers.
The pigment coating may be applied preferably by means of the so-called thin layer technology within the paper machine onto the base paper. Discribed is this technology in "Das Papier, 1991, volume 10 A, pp V120-124, Wochenblatt fur Papierfabrikation, 1993, volume 10, pp 390-393 and 1994, volume 17, pp 671-676".
The so-called differential coating method described in "Wochenblatt fur Papierfabrikation (1992), no. 6, pp 198-205" has proved to be particularly advantageous for the preparation of the thin films according to the invention. When this coating technology is used, a thin pigment coating is applied to one side of the paper web according to the invention using the thin layer technology mentioned above, and impregnation of the other side of the paper takes place at the same time in the roller nip (sump development).
The paper structure is altered by impregnation in a measurable manner in terms of its elasticity and brittleness/flatness, depending on the subsequent processing requirements. Moreover, the reverse side of the paper impregnated according to the invention should ensure a rapid and firm bond with the chipboard or MDF board to be laminated. This is only possible if the adhesive force and viscosity of the waterbased adhesives are tailored precisely to the surface properties of the chipboard and, above all, the furniture film. The physical and chemical nature of the surface of the thin decorative film may make an important contribution to this.
The impregnated and surface-pigmented thin films according to the invention may be smoothed on one side, on-line smoothing with a soft calender or machine calender having proved particularly advantageous for maintaining the opacity of the paper.
The single-side pigment-coated thin films according to the invention have good flatness and laminar strength and, on the pigment-coated side, have very good printing and lacquering properties. The reverse of the paper impregnated with synthetic resins permits rapid, problem-free bonding to fibreboard.
The thin film according to the invention is also suitable for other applications in which paper supports with excellent single-side printing properties or ability to be coated with waterborne or solvent-based media are desired, and where the reverse of the paper is also required to have very good bonding or laminating properties with different substrate materials. Wall linings, poster materials and special masking papers for packaging may be mentioned as examples.